Inhibition of prostaglandin synthesis constricts the ductus arteriosus in fetal lambs in utero. We administered the inhibitors, aspirin or indomethacin to 18 premature infants with patent ductus arteriosus, and assessed the effects clinically and by echocardiography (left atrial/aortic-root ratio). After aspirin (20 mg per kilogram, every six hours for four doses) the ductus closed permanently in one infant within 24 hours; in another, constriction occurred with clinical improvement, and the third did not respond. In five infants given 0.3 mg per kilogram of indomethacin, complete closure occurred within one day; two of them, who received three doses had an elevated serum creatinine for one week. In one infant the ductus reopened, requiring a second dose of indomethacin 11 days after the first. Ten infants received 0.1 mg per kilogram of indomethacin, and closure occurred within 24 to 30 hours in eight. One had a soft murmur for four days, and one did not respond to two doses of indomethacin. A murmur reappeared after three to seven days in three infants but only one required further treatment. In infants receiving a single dose of 0.3 mg per kilogram, or one or more doses of 0.1 mg per kilogram, renal function was unaltered.